Current limiting device

ABSTRACT

A device for limiting overload currents using a semiconductor element with at least one controllable semiconductor having an electron source (source), an electron acceptor (drain) and a control electrode (gate) controlling the electron flow, which device has characteristic curves typical of a field-effect transistor (FET). In the case of alternating voltage, two FETs are connected in series, in complementary fashion. Arrangement is provided for internally obtaining the control voltage required for driving the semiconductor element from at least part of the load current and/or from at least part of the voltage drop across the semiconductor element.

This application is a continuation of U.S. patent application Ser. No.08/596,240, filed Jul. 8, 1996, which was the National Stage ofInternational Application No. PCT/DE93/00824, filed Sep. 8, 1993.

FIELD OF THE INVENTION

The present invention relates to a current limiter for limiting overloadcurrents by means of a semiconductor element with at least onecontrollable semiconductor which has characteristic curves typical of afield-effect transistor (FET).

BACKGROUND INFORMATION

In the case of protective switchgear, such as power switches, motorprotection switches or automatic cut-outs, and in the case of automaticcircuit-breakers in general, it is desirable to sense overload currents,in particular short-circuit currents, quickly and to limit the overloadcurrents to minimum possible values and finally to disconnect theoverload currents. In the case of substantially mechanical automaticcircuit-breakers, for example automatic cut-outs, use is made ofso-called instantaneous trips, in which both the magnetic circuit isoptimally designed and a magnet armature, often a plunger armature,quickly and forcibly makes impact with the contact system. Nevertheless,it has not been possible in practice to achieve contact opening timesany shorter than about one millisecond. In this case, the short-circuitcurrent continues to increase unhindered until contact opening. Onlyafter contact opening is an arc produced, quickly conducted into an arcchamber and cooled at quenching plates, splitting the arc. The high arcvoltages which this generates have the effect of limiting theshort-circuit current and finally disconnecting it.

In the case of known automatic circuit-breakers, with a prospectiveshort-circuit current of 6 kA, cos phi=0.6 and psi=60°, it is scarcelypossible to obtain current time values any less than 4 ms for the risetime to the peak value, with a switching current of 4000 A, and anintegral of the square of the current over time of 30,000 A² s.

It has been suggested to use semiconductors for the current limitationin protective switchgear. This approach, however, is hampered orprevented in practice by various circumstances: 1) semiconductorelements generally have an inad equate current-limiting effect and aninsufficient permissible energy absorption; 2) semiconductor elementsgenerally have, in normal operation, a forward resistance of above 10milliohms at 16 A; and 3) semiconductor elements also generally have aninsufficient dielectric strength.

PCT International Application WO 93/11608 describes a power switch whichacts as a current limiter for limiting overload currents by means of asemiconductor element with at least one controllable semiconductor withan electron source (source), an electron acceptor (drain) and a controlelectrode (gate) controlling the electron flow, which current limiterhas characteristic curves of a field-effect transistor (FET), the loadcurrent flowing through the semiconductor element and, in the case ofalternating voltage, two FETs being antiserially connected. In thiscase, an external control voltage is provided.

SUMMARY OF THE INVENTION

The present invention is based on the object of developing a currentlimiter comprising a semiconductor element with at least onecontrollable semiconductor, which reduces the previously customarydisadvantages of semiconductor circuits to a technically usable extent.

The described object is achieved according to the present invention by acurrent limiter for limiting overload currents by means of asemiconductor element with at least one controllable semiconductor withan electron source (source), an electron acceptor (drain) and a controlelectrode (gate) controlling the electron flow of the load currentflowing through the semiconductor element. The current limiter of thepresent invention has characteristic curves typical of a field-effecttransistor and includes, if appropriate, as in the case of alternatingvoltage, two FETs connected antiserially, and means for internallyobtaining the control voltage required for driving the semiconductorelement from at least part of the voltage drop over the semiconductorelement and/or at least part of the load current flowing through thesemiconductor element. In this case, the control voltage required fordriving the semiconductor element is obtained from at least part of theload current which flows through the semiconductor element. The controlvoltage can also be obtained from at least part of the voltage drop overthe semiconductor element. The control voltage can also be achieved fromthe two measures in combination.

According to the present invention, the means for driving thesemiconductor element using the voltage drop may be respectively a drivecircuit connected to the drain and the gate of the semiconductor. Themeans for driving the semiconductor element using the load current canbe a current-to-voltage converter situated in the load current.

The drive circuit in the case of direct voltage may, in the simplestcase, comprise a resistor which is connected between the drain terminaland the gate terminal of an FET. A drive circuit in the case ofalternating voltage may, in the simplest way, be constructed such thatthe drain terminals of two antiserially connected FETs are connected tothe gate terminals via a valve and via a resistor, respectively. Such acircuit is suitable in connection with FETs of the enhancement type,that is to say with normally-off FETs, which have an n-channel, as astarting circuit, which makes it possible for the current limiter to runup into the operating state. Such a starting circuit is not required ifnormally-on FETs with n-channels, i.e., of the depletion type, are used.Depletion MOSFETs are particularly suitable in that case.

Also suitable as a drive circuit in the case of direct voltage is aconstant-current source which is connected between the drain terminaland the gate terminal. A drive circuit in the case of alternatingvoltage may be advantageously constructed such that the drain terminalsof two antiserially connected FETs (i.e., serially connectedcomplimentary FETs) are connected to the gate terminals via respectivelya valve and via a constant-current source.

Particularly suitable as the current-to-voltage converter for drivingthe semiconductor element using the load current is a transformer towhose secondary winding an element is connected for limiting the voltagein both directions of polarity, in particular two antiserially connectedzener diodes, whose outputs are connected to the gate terminals via arectifier circuit. Antiserially connected zener diodes are consideredhere to comprise all elements which have the effect of avoltage-limiting zener diode. In the case of alternating voltage,characteristic curves in the first and third quadrants of a diagram withthe drain-source current on the y-axis against the drain-source voltageon the x-axis are limited to a desired drain-source current.

The use of a transformer between two antiserially connected FETs as aninductance for limiting short-circuit current is described in EuropeanPatent Application No. 92 116 358.0 (see, FIGS. 4 and 7), assigned toSiemens AG. In this case, the transformer serves at the same time fortying in a control voltage to be applied externally.

In the case of the current limiter with a transformer designed as acurrent-to-voltage converter, the drive voltage is obtained from theload current, i.e., internally. Furthermore, the advantage is achievedthat the load current can be carried in a low-impedance primary windingwith few turns and that the secondary side can be of a high-impedancedesign with many turns, in order to derive a voltage for the drivingfrom the load current. It is in this case ensured by thevoltage-limiting element that the drain-source current on a curve withthe corresponding parameter of the gate-source voltage is limited in alow-loss manner.

It is advantageous to connect to a transformer, at its secondarywinding, a rectifier circuit and to connect a capacitor betweendirect-voltage potential points for the driving voltage. Morespecifically, in such an embodiment, there is connected to thetransformer at its secondary winding a rectifier circuit whosedirect-voltage potential points are connected on the one hand to thegate terminals of the FETs and on the other hand via a central tap ofthe primary winding and a capacitor is connected between thedirect-voltage potential points for the control voltage. The capacitormay be formed, if appropriate, by the gate-source capacitance of the FETif this is of adequate size. If voltage-limiting elements are used, itis advantageous for them to comprise zener diodes connected as a bridgerectifier whose output is connected to the gates.

The rectifier circuit may advantageously also be designed as a voltagemultiplier circuit.

Finally, the current-to-voltage converter may be a chopper with adownstream voltage multiplier, whereby the driving voltage can beobtained from the load current without a transformer.

The solutions according to the present invention and their advantagescan be further enhanced if the FETs are made from silicon carbide (SiC).In this case, the advantages of an FET made of silicon carbide and thoseof a current limiter complement one another.

A current limitation in connection with means for driving thesemiconductor element using a voltage drop or from a load current cangenerally be achieved by connecting between the gate terminal and thesource terminal an element with the effect of a current-limiting zenerdiode which is dimensioned such that the gate voltage of thesemiconductor element is set to a value at which the desired limitationof the overload current occurs.

In addition to obtaining the drive voltage internally, it is alsopossible to provide on the semiconductor element a drive device foradditional external driving, in which case corrective control voltagescan be applied externally. It is also possible, by external driving togenerate a voltage turning off the semiconductor element when apredetermined input signal is received. Such a current limiter then actsas a cut-out switch and can generally be constructed with semiconductorswhich have the described special properties. The current limiter may bedesigned as an integrated circuit on a chip, with discrete components orin a hybrid structure.

It may be advantageous for certain applications to arrange at least onemechanical switching contact in series with the semiconductor element. Arelatively simple switching contact without special quenching means thensuffices, since the current rise is limited by the current limiter. Onthe other hand, when it has been opened, the switching contact protectsthe current limiter against long-term overloading. This interactionpermits advantageous configurations.

PCT International Application WO 93/11608 describes a protective switchof an automatic circuit-breaker design with two antiserially arrangedFETs and a mechanical switching contact. In the case of this earlierprotective switch, designed as a power switch, a unit comprising relaysand switching contacts is connected in parallel with two antiseriallyconnected FETs. In such a switch, the switching contacts are arranged inseries with the interconnection of the FETs. However, the internalresistance of the semiconductor element at a certain control voltage hasa low value, and with increasing voltage over the working electrodesthere is a jump in the internal resistance, so that the triggeringelement of the relay is supplied with voltage and can initiate thedisconnecting operation.

The mode of operation of the current limiter with a mechanical switchingcontact arranged in series differs fundamentally. The switching contactmay be in engaging connection with a magnetic system directly orindirectly via an energy store, which magnetic system opens theswitching contact in dependence on the current limiter.

A particularly favorable interaction of the semiconductor element andthe magnetic system for the switching contact is achieved in anembodiment of the present invention in which the magnetic system has aprimary winding of low impedance, relative to the secondary winding, andon the one hand forms the transformer for obtaining a control voltagefrom the load current and on the other hand forms, with thelow-impedance primary winding, the excitation winding for the respectivemagnetic system whose armature is in operative connection with theswitching contact. In this case, multiple use of structural elements incombined arrangement is achieved. In particular, the armature air gapmay be bridged by an auxiliary yoke so that a well-closed magneticcircuit is produced for the current-to-voltage converter. The yoke isdimensioned in such a way that it already goes into saturation atcomparatively low currents. As a result, both the function of thearmature and that of the magnetic circuit for the current-to-voltageconverter remain unimpaired in practice.

Thus, for this purpose, the working air gap is bridged by an auxiliaryyoke which is dimensioned in such a way that it already goes intomagnetic saturation at currents which are less than the desiredoperating current for the magnet armature.

The semiconductor element may generally be used in an automaticcircuit-breaker, such as for example a power switch, an automaticcut-out or in a motor protection switch or other protective devices, asa current-limiting part with the function of a so-called limiter. Thesemiconductor element and the mechanical switching contact may be partof physically separate switching devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first, simplified exemplary embodiment of a currentlimiter in accordance with the present invention, in which the controlvoltage is obtained from the voltage drop over the semiconductorelement.

FIG. 2 illustrates a current limiter whose control voltage is obtainedfrom the load current.

FIG. 3 depicts a current limiter whose control voltage is obtained fromthe voltage drop over the semiconductor element.

FIG. 4 illustrates a current limiter whose control voltage is obtainedfrom the load current and from the voltage drop over the semiconductorelement.

FIG. 5 shows a the current limiter with a current-to-voltage converterbetween two antiserially connected FETs, with the control voltage beingobtained from the load current and a switching contact being used inseries with the current limiter.

FIG. 6 depicts a current limiter with a switching contact, in whichantiserially connected zener diodes are arranged as voltage-limitingelements in bridge connection on the secondary side of thecurrent-to-voltage converter and in which a capacitor is connectedacross the rectified voltage output for the control voltage on thesecondary side.

FIG. 7 diagrammatically illustrates the use of a voltage multipliercircuit for a current limiter in accordance with FIG. 6.

FIG. 8 depicts an exemplary embodiment of a current limiter with controlfrom the load current, which includes a current-to-voltage converterdesigned as a chopper with a downstream voltage multiplier.

FIG. 9 illustrates a current limiter with control voltage conversionfrom the load current.

FIG. 10 illustrates a current limiter with control voltage conversion inwhich the current-to-voltage converter is divided up by an auxiliaryvoltage conversion on the secondary side and a voltage conversion forthe gates.

FIG. 11 how additional external driving can be performed based on thecurrent limiter of FIG. 10.

FIG. 12 depicts an alternative design for a command element foradditional external driving according to FIG. 11.

FIG. 13 shows an exemplary embodiment of a current limiter in which atransformer as a current-to-voltage converter is in combination with themagnetic system for actuation of the switching contact.

FIG. 14 represents the full symbol for a normally-off FET, with ann-channel, and thereunder the corresponding abbreviated symbol as usedin the present application.

FIG. 15 illustrates operating characteristics of a current limiter inaccordance with the present invention.

FIG. 16 depicts a magnetic system for a current limiter whose workingair gap is bridged by an auxiliary yoke which is dimensioned in such away that it already goes into magnetic saturation at currents which areless than the desired operating current for the magnet armatures.

DETAILED DESCRIPTION

FIG. 1 illustrates a first exemplary embodiment of a current limiter inaccordance with the present invention in which a semiconductor element 1comprises field-effect transistors (FETs) 3. In the exemplaryembodiment, the FETs 3 may, corresponding to the representation in FIG.14, be understood as those of the enhancement type, which are normallyoff and have, for example, an n-channel. An abbreviated symbol for sucha FET is also shown in FIG. 14. Represented in FIG. 1 is a currentlimiter for alternating voltage, which with two polarities to beswitched operates with two antiserially connected FETs 3. For drivingthe semiconductor element, or the FET, the required control voltage isobtained from the voltage drop across the semiconductor element 1 byconnecting from the drain terminal the FET 3 a valve 4, for example adiode, in series with a resistor 5 which is coupled to the gate terminal6 of the FET 3. In the case of a current limiter adapted for alternatingvoltages, in which two antiserially connected FETs 3 are used (as shownin FIG. 1), a connection to the gate terminals 6 of the FETs isestablished between the drain terminals 7 via respective valves 4 andvia a resistor 5. The source terminals 8 of the FETs 3 are connected toone another.

If only a single-ended potential is to be switched, it suffices, on thebasis of FIG. 1, to have either the upper or lower FET 3 in connectionwith the corresponding valve 4 and the resistor 5. The source terminal 8may then be connected to ground.

In the exemplary embodiment of FIG. 1, with two antiserially connectedFETs 3 as the voltage-limiting element 9 there is a zener diodeconnected between the gate terminals 6 and the connection 10 of thesource terminals 8 of the antiserially connected FETs 3. The connection10 carries the load current.

The gate voltage of the antiserially connected FETs 3 is thus obtainedvia the valves 4 and via the resistor 5. The voltage-limiting element 9has the effect of limiting the gate voltage and consequently ashort-circuit current flowing at a maximum.

In FIG. 2 it is illustrated how the control voltage U_(S) is obtained asa function of the load current I, U_(S) =f(I). In FIG. 3 it isillustrated that the control voltage U_(S) can be achieved as a functionof the voltage drop U over the semiconductor element, i.e., U_(S) =f(U).In FIG. 4 it is illustrated how the control voltage U_(S) can beobtained as a function of the load current and as a function of thevoltage drop over the semiconductor element, i.e., U_(S) =f(I) and U_(S)=f(U).

In the embodiment according to FIG. 5, a mechanical switching contact 2is connected in series with the current limiter. The current limiteroperates with two antiserially connected FETs, which are interconnectedby their source terminals via a primary winding 12 of acurrent-to-voltage converter 11. A further essential feature in thisembodiment is that there is connected to the current-to-voltageconverter 11 on its secondary side, or at its secondary winding 13, anelement 14 limiting voltage in both directions of polarity, inparticular two antiserially connected zener diodes 15. Zener diodes 15connected on the secondary side limit the voltage on the secondary sideso that a voltage drop of only a few tens of millivolts occurs on theprimary side, owing to the transformation ratio of thecurrent-to-voltage converter 11. Load current also flowing through theFETs 3 on the primary side is consequently limited by the low-losslimitation of the voltage on the secondary side by thecurrent-to-voltage converter. This effect acts in concert with thelimitation inherent in semiconductors brought about by the specialdriving of the FETs 3. On the other hand, the transformation ratio ofthe current-to-voltage converter 11 permits a relatively high voltage tobe carried as the gate-source voltage to the primary side, whereby theON resistance is reduced. R_(ON) is obtained with large gate-sourcevoltages. Details of the effect are to be explained later with referenceto FIG. 15.

In the case of the exemplary embodiment according to FIG. 5, there isfurther connected to the current-to-voltage converter 11 on itssecondary side a rectifier circuit 16, which is connected on the onehand to the gate terminal 6 of the FETs 3 and on the other hand via acentral tap 18 of the primary winding 12. In the exemplary embodiment, acapacitor 19 performs a dual function as a storage capacitor. On the onehand, the capacitor 19 isolates the direct-voltage potential points 17for the control voltage. In addition, the capacitor 19 ensures that, inthe family of current-voltage characteristic curves of the semiconductorelement 1 with the antiserially connected FETs 3, it is not required ineach case to run up to the ON resistance between the parameter-dependantcharacteristic curves for the gate-source voltage in the first and thirdquadrants, but that even in the case of alternating voltage it ispossible to operate between the first and the third quadrants at the ONresistance. This is to be explained further with reference to FIG. 15.The capacitor 19 in the exemplary embodiment according to FIG. 6 servesin this second function. In said exemplary embodiment, thedirect-voltage potential points 17 of the rectifier circuit 16 are alsoprovided without the capacitor 19.

In the case of the current limiter according to FIG. 5, thecurrent-to-voltage converter 11 is not terminated in the customary wayby a resistor, but by the zener diodes 15 of the voltage-limitingelement 14. The tap of the direct-voltage potential points 17 has theeffect that on the primary side of the converter there is carried a gateauxiliary voltage, which supplements or substitutes the auxiliaryvoltage generation on the primary side, as has been explained withreference to FIG. 1. On the secondary side of the current-to-voltageconverter 11, for example at a zener voltage of about 9.1 V and aforward voltage of about 0.9 V over the zener diodes 15 in onedirection, a voltage totaling about 10 V is achieved. Thus, if a currentlarge enough to overcome the inductive resistance flows in the primarywinding 12, there occurs on the primary side, as a result of the 10 V onthe secondary side, a voltage corresponding to the transformation ratioof the current-to-voltage converter 11. For example, with atransformation ratio of 1 to 1000, a voltage of just 10 mV thereforeoccurs at the primary winding 12.

The operation of the circuit according to FIG. 5 will now be describedin further detail.

If there is a voltage at the connection terminals 20 and 21 of theautomatic circuit-breaker as the result of a switched-on load, thereflows via the valves 4, or the diodes, a current dependent on thepolarity of the alternating voltage. As a result of the voltage dropover the resistor 5 there is at the gate terminals 6 a potential whichis less positive with regard to the positive terminal voltage at 20, sothat at the FETs 3 there is an opening gate-source voltage and thedrain-source paths are brought into the ON state. The current flowingthrough the primary winding 12 of the current-to-voltage converter 11generates at the high-impedance secondary winding a voltage which, onreaching the zener voltage of the upper or lower zener diode 15, islimited to the zener voltage plus the forward voltage of the other zenerdiode, to be precise, in both directions of current flow correspondingto the alternating voltage. At the secondary winding 13, there isproduced in this case a virtually square-wave alternating voltage, whichgenerates by means of the diodes 22 for the rectification in the circuitof a full-wave rectifier at the capacitor 19 a direct voltage of thesize of the zener voltage of each of the zener diodes 15. This directvoltage is fed to both gate-source paths of the FETs 3, whereby thelatter are kept in the ON state, without continuing to require a voltagedrop over the resistor 5. In other words, current no longer flowsthrough the resistor 5.

In the exemplary embodiment according to FIG. 6, the voltage-limitingelement 14 takes the form of a bridge circuit comprising four zenerdiodes 15. In this circuit, there is no need for a current-limitingelement 9 on the primary side of the current-to-voltage converter 11. Aswitching contact 23 is again arranged in series.

The amplitudes of the alternating voltage in the secondary winding 13 ofthe current-to-voltage converter 11 can be kept smaller if a voltagemultiplier circuit 24 is connected downstream of the voltage-limitingelement 14, as is illustrated in FIG. 7.

For driving from the load current, the current-to-voltage converteraccording to FIG. 8 may be designed as a chopper 39 with a downstreamvoltage multiplier circuit 24. The voltage occurring at a resistor 55under load current is present at the chopper 39. For limiting thevoltage drop and to minimize the power loss, it is advantageous toprovide a voltage-limiting means 40. In the exemplary embodiment, thismay be the two diodes connected in antiparallel, which limit the voltagedrop over the resistor 5 to the forward resistance of the diodes.

In FIG. 9, the generation of the control voltage between the gateterminals 6 and the source terminals 8 of the field-effect transistors 3is diagrammatically reproduced. The generation of the control voltagemay be divided between a control voltage generation 25, in the case ofstarting, and an auxiliary voltage generation 26, as has been explainedin detail with reference to FIG. 5.

In FIG. 10, the construction of a current limiter with control voltagesupply 25 and auxiliary voltage supply 26 is illustrateddiagrammatically. The control voltage supply 25 may be designed as astarting circuit, so that the control voltage is then taken over inworking operation by the auxiliary voltage supply 26.

To be able to also drive the semiconductor element externally, anexternal drive device 41 according to FIG. 11 may be provided. Ifactuation contacts 42 are closed, the gate-source voltage isshort-circuited, so that a normally-off FET switches over into the offstate.

The external drive device 41 may also operate with semiconductorcontacts 43 according to FIG. 12.

In FIG. 13 there is illustrated on the one hand an advantageousrefinement of the arrangement according to FIG. 10 in which theauxiliary voltage conversion takes the form of a voltage multipliercircuit, and on the other hand a development according to which thelow-impedance primary winding is in operative connection with anarmature 27, which is to be brought into engaging connection with theswitching contact 23. Such a design is particularly inexpensive, sincethe current-to-voltage converter 11 and the magnetic system 36, whichopens the switching contact via the armature 27, are structurally andfunctionally combined. In addition, an energy store 38 of the latch typemay be provided. In this case, a high-impedance winding with many turnsmay be applied as a secondary winding 13 to the low-impedance primarywinding 12, driving the armature. In this case, a small auxiliary yoke37 (see FIG. 16) may close the magnetic circuit for the functioning ofthe current-to-voltage converter 11. The auxiliary yoke 37 isadvantageously dimensioned so that it already goes into saturation atcomparatively low currents, so that the function of the armature 27acting on the switching contact 23 is virtually uninfluenced. Thelow-impedance primary winding 12 may comprise few turns, for example twoto four turns, and a favorable voltage range for the auxiliary voltageconversion may be raised on the secondary side up to a desired voltagevalue by the voltage multiplier circuit. The voltage multiplier circuitcomprises the capacitors 28 and 19, the capacitors 19 at the same timeproviding the direct voltage for the driving of the FETs 3, and also thediodes 29, which in the circuit reproduced at the same time supply therectification.

The control supply 25 according to FIG. 13 shows one possibility forproducing a "fall-back" characteristic curve. The essential componentsfor this are the transistor 30 and the resistors 131, 132 and 133. Theoperation of this part of the circuit will now be described. If thecurrent-limiting action of the FETs 3 commences due to increasedcurrent, such as occurs for example in the case of a short-circuit, thevoltage across the terminals 20 and 21 increases. This voltage appearsat the bridge rectifier, which is formed by the diodes of the valves 4and the body (or "inverted") diodes 31 of the FETs 3. As is known, theterm "body diode" refers to the internal diode action, inherent in everyboundary layer, in particular a MOSFET, of the pn junction from sourceto drain. The voltage present at the bridge rectifier described is alsopresent at the series connection of the resistors 131 and 133, causing avoltage drop across the resistor 133 which switches the transistor 30 toa conductive state. The size of the resistor 132 can cause the turningon of a gate-source voltage which becomes smaller and smaller withincreasing voltage at the terminals 20 and 21 and consequently reduces aload current through the FETs 3.

The exemplary embodiment shown illustrates only one possibility forproducing a fall-back characteristic curve on the principles accordingto the present invention. As is known, any desired characteristic curvecan be produced with an operational amplifier.

In FIG. 14, the full symbol for an FET is reproduced in the upperrepresentation and the abbreviated symbol, as used in the presentdescription, is reproduced in the lower representation. The customaryabbreviations for drain, gate and source are used and the positivedirection of the drain-source current is indicated. The representationaccording to FIG. 14 shows an FET of the enhancement type, that is tosay a normally-off FET, which has an n-channel. In particular, themanner of representation according to FIG. 14 is to be understood as aMOSFET. It goes without saying that the reproduced circuits according toFIGS. 1 to 13 can also be realized by other corresponding components, inparticular by other FETs. For instance, if p-channel FETs are used, justthe customary polarity reversal has to be carried out. What is essentialis that characteristic curves such as those represented in FIG. 15 canbe realized, and hence that, for direct voltage, a maximum current canbe set irrespective of the voltage and that, for alternating voltage,such conditions prevail in two diagonally opposite quadrants. Thecircuits reproduced here by way of example on the basis of certain FETsare to be regarded in this general sense.

The operation of the current limiter will now be described withreference to FIG. 15.

FIG. 15 shows a graph of the drain-source current I_(DS), plotted on they-axis, and the drain-source voltage U_(DS), plotted on the x-axis. AnFET of the type described here, such as that explained with reference toFIG. 14, intrinsically has a characteristic curve 32 which, with anegative drain-source voltage, goes over into the characteristic curve33 of the body diode. The horizontal characteristic curves are obtainedwith a parameter of gate-source voltage and limit the drain-sourcecurrent in the case of corresponding wiring. At high gate-sourcevoltages, a steep ON resistance, R_(ON), is achieved. With an antiserialconnection of FETs, for the case of alternating voltage, a symmetricalmode of operation is achieved between the first quadrant and thirdquadrant, the characteristic curve 33 of the body diode no longer havingany effect. A circuit with a current-to-voltage converter, as described,achieves the effect of running up over the characteristic curve 35,which enters into the straight line for the physically predetermined ONresistance of the FETs used.

Running up for each direction of polarity of an alternating voltage isavoided in an antiserial arrangement of FETs if a capacitor 19 is usedas a storage capacitor, as described. The current-limiting action of theantiserially connected FETs then develops between a chosen horizontalcharacteristic curve with the corresponding gate-source voltage as theparameter in the first quadrant and one in the third quadrant inconnection with a transition of the characteristic curve 34 for the ONresistance. In this case, the area between the characteristic curve 32and a characteristic curve chosen on the left in the first quadrant actsas the loss saving, as can be seen illustrated by the product of thedrain-source current and the drain-source voltage. The possibilities ofthe principles described are further enhanced in a considerable way bythe use of FETs of silicon carbide. The semiconductor element 1, whichis connected in series with the switching device, can be realized in thevarious types of design in each case as a complete unit or partially asan integrated circuit. The current limiter can also have a wide varietyof applications without a switching device.

FIG. 16 shows a magnetic system 36 with a primary winding 12 and asecondary winding 13, which system has an auxiliary yoke 37 and anarmature 27. Such a magnetic system is advantageous for the structuralcombination of a current limiter with a switching device, as has alreadybeen explained.

We claim:
 1. A current limiter for limiting overload currents,comprising:a semiconductor element inserted into a load current path andincluding at least one controllable semiconductor structure, the atleast one controllable semiconductor structure including:a first FET,and a second FET connected antiserially with the first FET, each one ofthe first FET and the second FET having a source, a drain, and a gate;and means for internally obtaining a control voltage required fordriving the semiconductor element from at least one of at least a partof a voltage drop on the semiconductor element between the drain and thegate of one of the first FET and the second FET and at least a part of aload current flowing through the semiconductor element, the means forobtaining the control voltage including at least one of:a first drivecircuit for use during a d.c. voltage condition and including a constantcurrent source connected to the load current path, a second drivecircuit for use during the d.c. voltage condition and including a firstresistor connected between the drain and the gate of at least one of thefirst FET and the second FET, a third drive circuit for use during ana.c. voltage condition and including a second resistor and at least onevalve connected between the drain and the gate of at least one of thefirst FET and the second FET, and a current-to-voltage converterconnected in the load current path of the semiconductor element and usedto obtain the control voltage from at least the part of the load currentflowing through the semiconductor element; wherein the current limiterhas characteristic curves typical of a field-effect transistor.
 2. Thecurrent limiter according to claim 1, wherein means for obtaining thecontrol voltage includes the current-to-voltage converter, thecurrent-to-voltage converter including a chopper circuit having adownstream voltage multiplier.
 3. The current limiter according to claim1, wherein each one of the first FET and the second FET is formed fromsilicon carbide.
 4. The current limiter according to claim 1, furthercomprising a current limiting element connected to at least one of aconnection point between the gate terminal of the first FET and the gateterminal of the second FET and a connection point between the sourceterminal of the first FET and the source terminal of the second FET, thecurrent limiting element being dimensioned so that a gate voltage of atleast one of the first FET and the second FET is set at a value at whicha desired limitation of at least one of the overload currents occurs. 5.The current limiter according to claim 4, wherein the current limitingelement is a current limiting zener diode.
 6. The current limiteraccording to claim 1, further comprising a drive device coupled to thesemiconductor element, the drive device providing an additional externaldriving.
 7. The current limiter according to claim 6, wherein the drivedevice generates a voltage for turning off the semiconductor element inresponse to a predetermined input signal.
 8. The current limiteraccording to claim 1, further comprising at least one mechanicalswitching contact connected in series with the semiconductor element. 9.The current limiter according to claim 8, further comprising a magneticsystem coupled to the at least one mechanical switching contact throughone of a direct connection and an indirect connection using an energystoring device.
 10. The current limiter according to claim 9, wherein aprimary winding of the magnetic system has a lower impedance than thatof a secondary winding of the magnetic system, the magnetic systemacting as an excitation winding through the primary winding, themagnetic system including an armature in an operative connection withthe at least one mechanical switching contact.
 11. The current limiteraccording to claim 8, wherein the semiconductor element and the at leastone mechanical switching contact are respectively part of physicallyseparate switching devices.
 12. The current limiter according to claim1, wherein the semiconductor element is used as a current-limiting partin an automatic circuit-breaker corresponding to one of a power switch,an automatic cut-out switch, and a motor protection switch.
 13. Thecurrent limiter according to claim 1, wherein the control voltage isobtained automatically.
 14. The current limiter according to claim 1,wherein the means obtains the control voltage without utilizing anexternal arrangement.
 15. A current limiter for limiting overloadcurrents, comprising:a semiconductor element inserted into a loadcurrent path and including at least one controllable semiconductorstructure, the at least one controllable semiconductor structureincluding:a first FET, and a second FET connected antiserially with thefirst FET, each one of the first FET and the second FET having a source,a drain, and a gate; means for obtaining a control voltage required fordriving the semiconductor element from at least one of at least a partof a voltage drop on the semiconductor element between the drain and thegate of one of the first FET and the second FET and at least a part of aload current flowing through the semiconductor element, the means forobtaining the control voltage including at least one of:a first drivecircuit for use during a d.c. voltage condition and including a constantcurrent source connected to the load current path, a second drivecircuit for use during the d.c. voltage condition and including a firstresistor connected between the drain and the gate of at least one of thefirst FET and the second FET, a third drive circuit for use during ana.c. voltage condition and including a second resistor and at least onevalve connected between the drain and the gate of at least one of thefirst FET and the second FET, and a current-to-voltage converterconnected in the load current path of the semiconductor element and usedto obtain the control voltage from at least the part of the load currentflowing through the semiconductor element; a voltage limiting elementincluding a first zener diode and a second zener diode connectedantiserially with the first zener diode, wherein the voltage limitingelement limits a voltage applied thereto in both directions of polarity;and a rectifier circuit, wherein the current-to-voltage converterincludes a transformer, wherein the voltage limiting element isconnected to a secondary winding of the transformer, and wherein thefirst zener diode is connected to the gate of one of the first FET andthe second FET through the rectifier circuit and the second zener diodeis connected to the gate of another one of the first FET and the secondFET through the rectifier circuit; wherein the current limiter hascharacteristic curves typical of a field-effect transistor.
 16. Thecurrent limiter according to claim 15, further comprising a capacitor,wherein the rectifier circuit includes a plurality of direct-voltagepotential points and is connected to the secondary winding of thetransformer, each direct-voltage potential point being connected to acorresponding one of the gate of the first FET and the gate of thesecond FET and to a primary winding of the transformer through a centraltap, and wherein the capacitor is connected between the plurality ofdirect-voltage potential points.
 17. The current limiter according toclaim 15, wherein the rectifier circuit includes a voltage multipliercircuit.
 18. A current limiter for limiting overload currents,comprising:a semiconductor element inserted into a load current path andincluding at least one controllable semiconductor structure, the atleast one controllable semiconductor structure including:a first FET,and a second FET connected antiserially with the first FET, each one ofthe first FET and the second FET having a source, a drain, and a gate;means for obtaining a control voltage required for driving thesemiconductor element from at least one of at least a part of a voltagedrop on the semiconductor element between the drain and the gate of oneof the first FET and the second FET and at least a part of a loadcurrent flowing through the semiconductor element, the means forobtaining the control voltage including at least one of:a first drivecircuit for use during a d.c. voltage condition and including a constantcurrent source connected to the load current path, a second drivecircuit for use during the d.c. voltage condition and including a firstresistor connected between the drain and the gate of at least one of thefirst FET and the second FET, a third drive circuit for use during ana.c. voltage condition and including a second resistor and at least onevalve connected between the drain and the gate of at least one of thefirst FET and the second FET, and a current-to-voltage converterconnected in the load current path of the semiconductor element and usedto obtain the control voltage from at least the part of the load currentflowing through the semiconductor element; and a bridge rectifierincluding a plurality of zener diodes and a plurality of direct-voltageoutputs, each direct-voltage output being connected to a correspondingone of the gate of the first FET and the gate of the second FET, whereinthe current-to-voltage converter includes a transformer having asecondary winding coupled to the bridge rectifier; wherein the currentlimiter has characteristic curves typical of a field-effect transistor.19. A current limiter for limiting overload currents, comprising:asemiconductor element inserted into a load current path and including atleast one controllable semiconductor structure, the at least onecontrollable semiconductor structure including:a first FET, and a secondFET connected antiserially with the first FET, each one of the first FETand the second FET having a source, a drain, and a gate; means forobtaining a control voltage required for driving the semiconductorelement from at least one of at least a part of a voltage drop on thesemiconductor element between the drain and the gate of one of the firstFET and the second FET and at least a part of a load current flowingthrough the semiconductor element, the means for obtaining the controlvoltage including at least one of:a first drive circuit for use during ad.c. voltage condition and including a constant current source connectedto the load current path, a second drive circuit for use during the d.c.voltage condition and including a first resistor connected between thedrain and the gate of at least one of the first FET and the second FET,a third drive circuit for use during an a.c. voltage condition andincluding a second resistor and at least one valve connected between thedrain and the gate of at least one of the first FET and the second FET,and a current-to-voltage converter connected in the load current path ofthe semiconductor element and used to obtain the control voltage from atleast the part of the load current flowing through the semiconductorelement; at least one mechanical switching contact connected in serieswith the semiconductor element; a magnetic system coupled to the atleast one mechanical switching contact through one of a directconnection and an indirect connection using an energy storing device,wherein a primary winding of the magnetic system has a lower impedancethan that of a secondary winding of the magnetic system, the magneticsystem acting as an excitation winding through the primary winding, themagnetic system including an armature in an operative connection withthe at least one mechanical switching contact; and an auxiliary yokethat bridges a working air gap of the magnetic system, the auxiliaryyoke being dimensioned in such a way that the auxiliary yoke enters intoa magnetic saturation condition at currents which are less than adesired operating current for the armature; wherein the current limiterhas characteristic curves typical of a field-effect transistor.